Sound recording



Sept. 28 1926. 1,601,078

D. M KENzlE SOUND RECORDING Filed Oct. 16. 1924 f/yz L0 |.I L2 L5 L4 L5 L6 L7 L6 L9 2.0 2.\ 2.2 23

Positive contrast factor -y+ Log. expoaure Mme/2f.- po/la/d/ lavlmz/e Patented Sept. 28, 1926.

UNITED STATES PATENT OFFICE.

DONALD MACKENZIE, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y A CORPORATION OF NEW YORK.

SOUND RECORDING.

Application filed October 16, 1924. Serial No. 743,904.

This invention relates to sound recording and particularly to the making. of photo graphic sound records;

An object of the invention is to provide a method of, and means for, producing effioient photographic sound records, such as are used, for example, in talking motion pictures.

Photographic sound records may be made by any of the well known light-modulation methods. Most of the usual methods involve the transformation of sound variations into electrical variations, which are in turn made to cause pro ortional variations in the quantity of light ocused on a moving sensitized film through a slit in a screen.- The exposure of the film, which is the in tensity of illumination multiplied by the' time of exposure, will therefore, vary in accordance with the sound variations. The exposed film is developed in a suitable manner to form the negative sound record, the record of the sounds being in the form of a F series of striations of varying opacity on the film. From this negative film a positive record is made by the contact process or other means. This positive record, when suitably developed, is used to reproduce the original sounds recorded. One inethod of reproduction comprises subjecting. the continuously moving positive film to rays of light transmitted through a narrow slit of constant width. The transmitted ,light, varying in accordance with the striations on the film, is focused on a cell sensitive to light, such as a photo-electric cell, connected in an electric circuit. The variations of light will cause variations of the electric current through the cell, which variations will be proportional to the recorded sound waves. The varying current is cgused directly or indirectly to act upon suitable telephone apparatus, such as a loud speaker, to reproduce the original sounds recorded.

Due to the limitations of the apparatus used in recording the sounds.'there will be a considerable loss in the volume of useful sound (or increased amplification necessary to secure the same volume) in transferring thesound Waves to a photographic film. The volume range of the final sound wave record may be reduced, for example, because of the overload limitations of the recording apparatus used or because of interference with nolses inherent in this apparatus. It IS of course, desirable, in order to avoid interference due to undue amplification of vacuum tube noises and noises innerent 1n photoelectric cells, to get as much light through the film as possible. This means avoiding the production of sound record films havlng too great a mean density. In accordance with the invention, it has been found that the mean density of the final sound record may be decreased, and, therefore, the volume of useful sound initially obtainable from sound record films greatly increased, if the positive and negavtive sound records are both made on films having thereon a rich fine grained emulsion having inherently great contrast, such as is used, for example, in standard positive mo-- tion picture film, and if the development of both films is so chosen as to give 'unit contrast.-

' The invention will be clear from the following detailed description read in connection with the accompanying drawing in which Fig. 1 shows the characteristic curves of standard positive and negative motion picture emulsions, and Fig. 2 shows how the transmission of a film varies with the positive contrast factor.

In Fig. 1 are shown characteristic curves of standard positive and negative photographic film emulsions, as determined from sensitometer tests. As is well known, standard positive film has a rich fine-grained.

emulsion having inherently great contrast, and is commercially referred to as a slow film, while standard negative photographic film has a thin coarse-grained emulsion and is commercially referred to as a fast film. However, the relative speed of films as given by the commercial designations has reference more to the lower light intensities, and

the deposit transmits an intensity ,I, then we may write Transmission as- Opacity as- Density as D=logO==log Referring to the curves, it will be noted that over the portion AB of the negative emulsion curve, the density gradually increases with exposure. In this range the film will be under-exposed. The straight line B0 of the negativeemulsion curve represents the range Where the density shows a uniform rise for equal logarithmic increases in exposure. This is termed the range of correct exposure. Over the portion CD of the curve. density increases with exposure at a diminishing rate. This corresponds to the range of over-exposure. Similarly, for the positive emulsion curve there is a range of under-exposure AB, a range of perfect exposure BC and a range of over-exposure CD The tangents of the angles, 6 and 0', which the straight line portions BC and BC of the negative emulsion curve and the positive emulsion curve respectively make with the exposure axis, are known as the negative 'contrast factor and the positive contrast factor re spectively. Therposit ive and negative con trast factors are usually designated as y+ and y-, respectively. The values of log.

exposure at which the characteristic curves begin to be straight are independent of the development, but the steepness of the straight line portions. and therefore the contrast factors 31- and 'y+, increase as the time of development increases. In the I curves as shown in Figpl, the time of development of the negative emulsion was 8.5 minutes" and that for the positive emulsion 2 minutes, this corresponding approximately to 3 :0.60 and y+=1.00. It is evident therefore that unit contrast requires a much longer time of development for the negative emulsion than for the positive emulsion. It will be noted that both curves begin'to be straight at approximately the same exposure. For perfect sound transfer, both positive and negative emulsions must be exposed in such a manner as not to exceed their straight line range.

When asound wave is recorded on a negative and the latter printed, the print should be such as to give faithful contrast reproduction. Now, it is well known in hotography, that for exact reproduction o contrast, the development-of the positive print must be such as to give a contrast factor equal to the reciprocalof that prevailing in the negative from which the positive is printed, that is, the product of the negative and the positive contrast factors must be equal to unity. Also, the exposure of thepositive must insure that the negative record is printed throughout within the straight line range of the positive emulsion, and the least transparent region of the negative should, for greatest transparency of the positive print at the lowest density of the positive emulsion consistent with the straight line requirement,

The length of the straight line portion of the characteristic of an emulsion is called the latitude of the material. This latitude varies a great deal with different emulsions being less in contrasty, fine-grained emulsionsysuch as are used in standard positive film, than in coarse-grained thin emulsions, such as are used in standard negative films. In the particular emulsions used for which curves are shownin' Fig. 1, the logarithmic exposure latitude for the negative emulsion is 2.0 and'that for the positive emulsion is 1.2. ,The transmission range of the negative film is obviously the ex posure range of the positive printed from it.

the negative recor is the greatest allowable, namely 220, the development of the Therefore, if the logarithmic range used in negative must be such as to compress this logarithmic exposure range of 2.0 into a logarithmic transmision range of 1.2 or less, that is to say, the negative must be developed for a contrast factor of 0.6. If a narrower range of negative exposure ob- .tains, a higher negative conti'ast may be permitted, In general, if the positive range is log R+ and the negative range log R, the development of the negative should be suclii that the contrast factor does not excee Now, unit contrast may be obtained with negative emulsion if the development is prolonged suificiently. However, it has been found that if the development of the negative emulsion is carried far enough to ohmay be tain that result, chemical fog will be obtained on the negative to such an extent as to make the negative record worthless. This due in part at least to the deterioration of the developer due to the increased opment is 16.5 t. u. This loss cannot be this invention that if both the negative sound.- record and the positive eprint thereof are time used in development. Because of this chemical fog in development, the negative development must be limited to 1 -:O.7 5. The corresponding positive development to give faithful contrast reproduction will be Fig. 2 shows the variations of the transmission at the lower end of the straight line portion of the ,standard positive emulsion characteristic with various values of the positive contrast factor, as determined from sensitometer tests. If T isthe average transmission of the positive, the transmission at the lower end 0 the straight line is-2 T. It will be noted that the transmission falls off gradually as the positive contrast factor increases above unlt As noted above, because of the risk 0 fog, the negative devellimi-tedto y- 0.75 and for unit contrast the corresponding positive development is y+::1.33. Under such conditions, the average density of the positive film will be such as not to give an efiicient transmission of li ht in the reproduction process. The transfer attenuation or loss of volume under such conditions has been measured and found to be in transmission units about decreased by increasing the light intensity used. It has been determined experimentally that doubling the light strength will not reduce the loss while quadrupling the intensity of the light will increase this loss by 1.0 t. u.

It is likewise undesirable to increase the area of ex osure by widening .the aperture throu h w ich the light is directed on the ecause the limiting frequency for a ven speed is inversely proportional to the aperture width. -Therefore, this would result in an undesirable limitation of the frequenc range. At present, at. a film speed of 15 per seconda tone of 7000 cycles is and printed by suitable readily recorded and this limit should P g aphic handling, not be lowered. I

Now it has been found in accordance with made on a rich fine-grain emulsion having inherently high contrast, having a character-.

istic. such as shown for the positive emulsion in Fig. 1, the positive being printed from the negative in the customary manner, and

both films developed to unit contrast, the

trast nature of such ,an emulsion makes it possible to obtain unit contrast without prolonged development. It has been determined experimentally that the transfer attenuation under such conditions is 14.5 t. u., as compared with 16.5 t. u. when standard negative emulsion is used for making the negative. Thus, an increase of 2.5 transmission units is obtained.

An additional advantage is obtained by using standard positive emulsion in making the negative in that such an emulsion .has a higher resolving power than standard negative emulsion, and, therefore the successive striations do not merge into each other so much. Thus, improved definition in the final record is obtained.

What is claimed is:

1. The .methodof making photographic sound records which comprises making a negative record of the sounds on a rich, finegrained photographic emulsion having inherently great contrast, developing said negative record for unit contrast, printing a positive record from said negative on an emulsion having the same characteristic as said first mentioned emulsion, and developing said positive record for unit contrast. 7

2. The method of increasing the volume of sound obtainable from a given light intensity working through a photographic sound record which consists in making the negative sound record on a rich, fine-grained photographic emulsion having inherently great contrast, and printing therefrom a positiverecord on an emulsion of like characteristic,

and developing said negative sound record and said positiverecord-to give unit contrast.

3. In the art of photographically recording sound variations, the method comprising using a rich,

emulsion having inherently great contrast for making a negative record of the sound fine-grained photographic variations, using an emulsion of like charac-.

, DONALD MACKENZIE.

of the 

